DESCRIPTION (adapted from the application) Non-alcoholic steatohepatitis (NASH) is a disease of emerging clinical significance. The risk factors for NASH include female gender, non-insulin dependent diabetes, obesity and hyperlipidemia. In NASH, the fat that accumulates in the liver is primarily triglyceride (TG) and three sources potentially contribute to this lipid are fatty acids (FA) derived from the diet, those originating in the adipose tissue (FFA in the plasma), and FA newly synthesized in the liver via process called de novo lipogenesis. The origin of the fat that accumulates in the liver has not been extensively investigated previously due to the technical challenges of studying de novo lipogenesis and the limitations of using radioactive isotopes in humans. Recent advances in gas chromatography/mass spectrometry and stable (non-radioactive) isotope methodology now make it possible to study hepatic TG metabolism in vivo. The hypothesis to be tested is that de novo lipogenesis contributes substantially to hepatic TG found in NASH. Further, it is hypothesized that plasma-derived FFA will contribute quantitatively less to the fat stored in hepatocytes and more to the TG that is exported from the liver in lipoproteins. Patients with persistently elevated liver enzymes of uncertain etiology, who are being considered for liver biopsy, will undergo a 5-day, stable-isotope infusion of labeled FA and precursors of FA, preceding the scheduled biopsy. Liver biopsy tissue (100 mg) will be analyzed to determine its biochemical content (TG, cholesterol, phospholipid and FFA), the composition of FA within these fractions, and the enrichment of labeled FA in the tissue (the sources of these FA). Control subjects will be aged- and sex-matched individuals undergoing surgical treatment for obesity who will have an identical isotope infusion before surgery. These methods will be used to accomplish the specific aims: (1) to quantify the concentration of the various lipids in NASH liver samples and samples from obese control subjects; (2) to determine the sources of FA used for lipid synthesis, and the turnover of these lipids in NASH patients and controls; and (3) to determine whether there is a difference between NASH patients and controls with respect to the composition of FA within liver tissue. Liver samples will be graded histologically and the stage of NASH documented semi-quantitatively. Computer tomography will be used to quantify liver size and abdominal visceral fat; ultrasound will also be performed. The results of all of these measurements will be analyzed to determine their relationship with hepatic lipid content. NASH will become more clinically important in the future as the incidence of obesity and diabetes continue to rise in the United States. In combination with the clinical data obtained, an understanding of the contributions of FA sources to liver TG will aid in the development of future treatment strategies for this disease.